Approaches Of Sparing Lung In Sbrt For Treating Tumors In Lungs With Volumetric Arc Therapy

Sparing lung in Stereotactic Body Radiotherapy (SBRT) for treating tumors in lungs is critical to reduce the radiation induced toxicities, such as radiation pneumonitis and late fibrosis. A retrospective study was performed with a new combination of approaches to improve lung sparing in SBRT plans with VMAT and Monte-Carlo dose calculation algorithm. Twenty two SBRT lung plans were optimized using traditional LINAC MLC based approaches: an uniform dose distribution in the target with an average P-IDL (prescription isodose line) of (88.8±1.9)% (The error bar of all the data is the 95% confidence interval (CI)) and clinically delivered. The plans were re-optimized using a new combination of approaches with variation of P-IDL and evaluated with: 1. R50%, the ratio of 50% prescription isodose volume to the plan target volume (PTV); 2. V20, the volume of lung within 20Gy; 3. PCI, the Paddick comformity index; 4. D2cm, the maximum dose at 2 cm from PTV in any direction; 5. Dmean, the mean dose in total lung volume. It was found that the optimal P-IDL was in the range of 75%∼80%. With the new optimization strategies and the optimal P-IDL, the average PCI was increased by (10.3±2.1)%; the average R50%, V20, D2cm and Dmean were decreased by (30.2±4.1)%, (27±5.4)%, (13.4±4.3)% and (16.6±2.3)%, respectively; the dose distributions in the targets also were changed from concave to convex. The new set of optimization approaches can significantly improve the lung sparing in SBRT VMAT plans with Monte-Carlo dose calculation algorithm for treating tumors in lungs while increasing the dose to the target. The new plans were more conformal in both high and intermediate dose regions. The optimal P-IDL was found in the range of 75-80%.